Oxygen and moisture barriers are commonly used for packaging components in a wide variety of industries, e.g., ranging from food packaging to electronics. The requirements vary significantly for permeation, cost, flexibility, weight, and transparency, among other considerations. Organic light emitting diodes (OLEDs) and flexible photovoltaics (PV) have some of the most demanding specifications for water vapor barriers, needing to couple low permeation rates with good flexibility and transparency.
For PV in general, low cost and scalability are important factors. Also PV is often subject to temperature fluctuations and harsh weather conditions, in addition to the expected ultraviolet (UV) exposure. Presently, to meet warranty specifications, glass and metal foil barriers are commonly used. However, these barriers may be heavy, expensive, and/or are not readily used for roll-to-roll manufacturing processes (metal foil barriers cannot be the front-sheet in roll-to-roll manufacturing). For weight reasons as well as benefits in cost and scalability through roll-to-roll manufacturing processes, a transparent, flexible barrier that can supplant existing approaches (e.g. glass superstrates) is highly desired by the PV industry.
More specifically, OLEDs and organic photovoltaics need to be packaged using a material with a low water vapor transmission rate (WVTR), in some cases on the order of 10−6 to 10−8 g/m2/day. For other PV applications, materials are generally not considered to have meaningful barrier properties unless their WVTR is at least about 10−4 g/m2/day, and more desirably at least about 10−6 g/m2/day.
The WVTR numbers typically quoted are at ambient temperatures or 38° C. In PV applications, damp heat conditions of 85° C. at 85% relative humidity (RH) are typically used in qualification tests, such as IEC (International Electrochemical Commission) 61646 and IEC 61215. Such conditions significantly increase permeation rates of the barriers and may irreversibly damage expensive structures. Therefore, it is often desired to be able to separately test individual barriers under a variety of environmental conditions.
Several techniques have been developed for measuring WVTRs of barrier materials below 10−4 g/m2/day, however, no generally accepted test standard exists. Of the experimental techniques with ultra-low sensitivities, the electrical calcium test offers high throughput capabilities, easily controlled environmental conditions, and in-situ measurements capable of studying steady state WVTRs and transients. The calcium degradation test method uses a thin calcium (Ca) layer to scavenge water that passes through a test barrier. Determining the amount of Ca degradation depends on the absorption of nearby water molecules by the Ca metal film to form CaO or Ca(OH)2. The electronic detection method is based on the transition of the Ca film from a highly conductive metal to a non-conductive oxidized state. The amount of Ca remaining can be calculated from the resistance measurements (using an assumed or a measured bulk resistivity for Ca), and the WVTR can be calculated from the rate of change in conductance with time. To aid in the research and development of such barriers, a high-throughput, electrical calcium method that can measure many barriers in parallel under a variety of test conditions (high temperature and relative humidity) with high accuracy is desirable.
Another limitation in measuring permeability using the electrical calcium test is the need for a better edge seal. Universally, in Ca electrical testing to date, a UV curable epoxy is used as a sealant for the edge seal. Numerous epoxies and other adhesives deleteriously interact with the calcium to varying degrees. While outgassing is possible for certain adhesives, great care needs to be taken not to harden them prematurely. Furthermore, UV curable adhesives are found to sometimes not be fully cured or when fully cured to become brittle, thereby increasing the chance of delamination. When measured at elevated temperatures, thermal expansion further increases the risk of delamination in epoxies. Furthermore, in the best epoxy edge seals, while edge permeation relative to many material systems is low, the ability to make extremely long-lived measurements is limited. Typically, only about 200-300 hr of permeability testing at about 38° C./90% RH (relative humidity) is achievable due to poor availability of good edge seals.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.